The present invention relates to a sealable, transparent multilayer film which is composed of a base or support layer comprising a propylene polymer and a sealable surface layer having a low minimum sealing temperature.
Films of this kind are used as packaging films in horizontal forming, filling and sealing machines. In the packaging process, the film which enters into the machine as a flat web is bent about its longitudinal axis so as to wrap the product to be packaged and is shaped into a tubular casing. The sealable surface layer forms the inside of the tubing. The edge zones of the film are placed one on top of the other in such a way that inside is in contact with inside. By the application of heat, a strip-shaped sealed seam (fin seal) is then produced, which projects from the tubular package and extends parallel to the longitudinal axis of the tubing. For portioning the product which has been filled in the forming individual packages, sealed seams are produced which extend, in mutually spaced arrangement, transversely to the longitudinal axis of the tubing and the tubular casing is severed in the region of these transverse seams transversely to the longitudinal axis of the casing.
It has been found that customary polypropylene-based films are hardly suitable for this application. Especially in modern, high-speed forming, filling and sealing machines, in which the film is conveyed in the horizontal direction at a speed exceeding 40 m/min, machine runability and slip properties of the film must be particularly good.
U.S. Pat. No. 4,297,415 has disclosed a sealable, transparent polyolefinic multilayer film which is intended for use as a packaging film in high-speed forming, filling and sealing machines. The film is composed of a base layer comprising a propylene polymer and two sealable surface layers comprising copolymers of ethylene with propylene or of ethylene with butene-1. If this film is to be corona-treated to improve printability it must, however, comprise further additions of slip agents, which will adversely influence its optical properties, such as gloss and degree of turbidity.
The sealable, transparent polyolefinic multilayer film according to U.S. Pat. No. 4,502,263 is formed of a base layer comprising a propylene polymer and at least one sealable layer comprising a polyolefin resin, a low molecular-weight resin which is compatible with the polyolefin resin, a propylene homopolymer and a polydiorganosiloxane. It is also intended for use as a packaging film in horizontal forming, filling and sealing machines and actually shows good processing characteristics in high-speed packaging machines. Although this multilayer film meets the technical requirements demanded of a packaging film for use in a packaging machine, handling of the film gives rise to problems, since it has a relatively smooth surface. For example, when these films which are present on 4 to 6 m wide original rolls, are cut to smaller widths, an increased oscillation results. Due to this lateral variation in the rewound film of reduced width, the new roll does not have the exact width, corresponding to the width of the cut film web. In addition, there is a strong tendency in the wound up film that film layers lying one on top of the other slip laterally from the roll, parallel to the longitudinal axis of the roll, a phenomenon which is known as "telescoping." As a result of telescoping, the edges of the film are no longer exactly superposed in the roll, but are, to a slight distance, laterally displaced with respect to each other. Owing to these difficulties, the machine speed must be drastically reduced in the cutting and winding process, which considerably increases the cost of film conversion. To obviate this difficulty, a silicate having the shape of platelets is added to the sealable surface layer, according to German Offenlegungsschrift No. 3,331,983.
It has been found, however, that these films can only be used in applications, in which they should be absolutely transparent to provide an unobstructed view of their contents. However, it is frequently required to apply a printing to the packaging film. In these cases, the film must be subjected to a corona-discharge treatment to render it printable. By this treatment, the surface tension of the film is increased and adhesion between film surface and printing ink is thus improved. This pretreatment has, however, the great disadvantage that the films so treated lose their good running characteristics in high-speed packaging machines.